Star streaks?

I was in the mountains last night for New Year's. Finally above Taiwan's smogline, I was actually able to see -- stars! Seeing them again for the first time, I was amazed at how "fidgety" they all seemed to be. No matter how hard I stared at them, and in fact, the harder I stared at them, they seemed to twitch and in some cases even change position. ("No wonder there are so many UFO sightings," I mumbled.) Thinking with such wonder about starlight, I recalled that the starlight we see is in fact old news: the light currently being emitted by the stars has not yet reached earth from so far away, so when we look up, we only see stars' old, well traveled photons.

INSCITIA:

If the earth is moving and if starlight reaches us in a stream of photons, why don't we see stars as streaks across the sky? Indeed, isn't the North Star so very bright just because earth 's axis moves in line with it, and therefore more photons from the star can hit us directly, without any terrestrial shift?

Imagine a man spraying a hose to the side while running: the water bends back as he runs forward. How or why doesn't the same go for starlight? Or imagine taking a photo of moving cars at night, with a long shutter exposure: as the photons collide into the camera's retina, the moving lights form streaks in the final photo. In both cases, the dynamics are reversed (the stars spray and are stationary, while the earth gets sprayed [with photons] and is in motion), but I think the analogies holds.

COGITATIO:

I assume the answer has to do with the immense distances of the stars from earth, and that from that far out starlight somehow "equalizes" around its originating point, so that we see the mean amount of light and recognize it as a star. In other words, the stars we see are like visual "statistical averages" of photons, and they appear to contract into one star. Since the only thing consitstently producing light is the star, it is the only thing that will reach our eye on a "statistically mean" frequency. Any twitching by the stars, then, would just be the batches of photons outside the star's statistically mean core.

RESPONSUM PRIMUM:

As of 4 December 2006, my co-workers quickly disabused me of my ignorance on this one. "It's distance versus speed, dude," they said. They reminded me that if we leave a camera shutter open a while at night, we will indeed see streaks. I am supposed to understand that because the stars are so very far away, we simply can't perceive their motion. It's like sitting on a train watching the far mountains "stand still" while the nearby grass, fences, ground, etc all zoom by.

This explanation is obvious and simple. Even so, I can't shake my sense of curiosity - which I'll say is so inchoate as to amount to ignorance - as to WHY such a phenomenon occurs. It still seems there must be some kind of perceivable alteration of the stars based on our terrestrial motion, much as the sun changes appearance just in relation to our orbit of it. "You can lead a fool to wisdom..." I suppose? I accept the explanation, but something elemental still tantalizes and eludes me at a deeper level about this phenomenon. Hmmm...

RESPONSUM SECUNDUM, care of "the other e.b.":

We can see the stars move due to terrestrial motion, paralax, dude.

Some practical explanation. Close an eye (any eye) and raise your thum to a fixed point on a far wall... now switch eyes, your thumb appears to have moved relative to the fixed point, right? Yeah baby, paralax.

And if you think about it this would work for "closer" objects as the earth moves around the sun. For instance durring summer a certain relatively close star would apper distance x away from a "fixed" star (one whose distance is so astronimically far that it appears unmoving even relative to our solar orbit). However, as the year progresses to winter the earth moves around to the other end of its orbit (like "switching eyes" in the previous example) and we now see the "close" star from a slightly different angle and it now looks as though it has traveled to a distance y from it's "unmoving" neighbor.

This only works for stars that are on the order of "astronomical units" away from us (one astronumical unit (au) is the distance from the earth to the sun, or thereabouts [using a base line of "1 AU" made some of the math easier given that, at the time, astronomers had no idea what the true distance was])

So, if we know the distance the earth has moved (roughly 2 au's), and we can see how much the "close" star has "moved" agains the relatively fixed background we can then extrapolate a distance using simple geometry (the earth's orbit is a base of a right triangle and the angle can be deduced from the amount the "close" star travels, apply the pathagorian theorem and there you have it).

pax